Method and device for separating a mixture of fluids

ABSTRACT

A mixture of fluids is separated into at least two phases, one of which has a higher density than the other, passing the mixture through a normally horizontal supply pipe, by creating a stratified flow in the supply pipe, by passing the mixture through an inclined pipe, whilst maintaining a stratified flow in the inclined pipe, by extracting fluid with lower density (“lighter phase”) via a first discharge system and fluid with a higher density (“heavier phase”) via a second discharge system, wherein the interface between the lighter phase and the heavier phase is monitored in the inclined pipe by a level controller means that varies the flow of the fluid of higher density to keep the interface between set levels. The invention also provides a device for employing this method.

The present invention relates to a method and device for separating amixture of fluids, which are not completely mixable. Such mixturesappear, e.g., in the recovery of natural resources from oil and gaswells. The products of such recoveries can include mixtures of gas, oiland water. These mixtures may comprise three phases. Depending onpressures and other physical circumstances the mixtures recovered maycomprise two phases, a hydrocarbon phase containing oil and optionallydissolved gas, and water. It also occurs that hardly any gas is presentso that there is a two-phase mixture.

It is desirable to separate the water phase from the hydrocarbon phasebefore transporting the valuable products further. Thereto devices havebeen developed to separate such mixtures into the respective phases, andremoving the water phase.

Russian patent publication No. 2 077 364 discloses a device forseparating a mixture of fluids into three phases, gas, liquid of lowerdensity and a liquid of higher density, having a feed inlet, a gasoutlet, and outlets for the liquid phases. During operation of thisdevice, a mixture of gas, low-density liquid and high-density liquid issupplied to the feed inlet of the separation device. The mixture passesupwards through an upwardly inclined supply pipe towards the inlet of adownwardly inclined pipe. In the upper end of the downwardly inclinedpipe, gas is separated from the liquids that fall downwards towards thelower end of the downwardly inclined pipe. Gas, lighter liquid andheavier liquid are separately removed from the device via the respectiveoutlets.

A disadvantage of the known separation device is that turbulence isgenerated in the upwardly inclined feed pipe and that counter-currentflow prevails in the downwardly inclined pipe, which phenomena adverselyaffect the separation efficiency of the device.

European Patent Application No. 1044711 describes a device forseparating a mixture of fluids into three phases; gas, lower-densityliquid (“lighter liquid”) and higher-density liquid (“heavier liquid”),having a feed inlet, a gas outlet, an outlet for the lighter liquid andan outlet for the heavier liquid. The device comprises:

-   -   a normally horizontal supply pipe with the feed inlet at its        upstream end;    -   an inclined pipe having an inlet at its upper end that is        connected to the outlet of the supply pipe and having a closed        lower end;    -   a gas discharge system comprising a gas riser having an inlet        that is located in the gas-filled space and an outlet that is in        fluid communication with the gas outlet of the device;    -   a discharge system for lighter liquid having an inlet that is        located in the lighter liquid-filled space and an outlet that is        in fluid communication with the outlet for lighter liquid of the        device; and    -   a discharge system for heavier liquid having an inlet arranged        below the bottom level of the supply pipe and an outlet that is        in fluid communication with the outlet for heavier liquid of the        device,        wherein the diameter of the supply pipe is selected such that        during normal operation the velocities of the liquids are below        a pre-determined value, wherein the ratio of the length of the        supply pipe to its diameter is larger than 10, and wherein the        slope of the inclined pipe is selected such that during normal        operations a stratified flow is maintained in the inclined pipe.

One of the advantages of this device over the device of the Russianpatent is the maintenance of a stratified flow. However, the dischargesystems, especially the discharge system for the heavier liquid iscomplicated. Although the design can operate at a wide range-ofoperating conditions, it does not provide for any active flow control.

It is an object of the present invention to overcome thesedisadvantages.

Accordingly, the present invention provides a method for separating amixture of fluids that are not completely mixable into at least twophases, one of which has a higher density than the other, which methodcomprises:

-   -   passing the mixture through a normally horizontal supply pipe        with a feed inlet at its upstream end and an outlet at its        downstream end;    -   subsequently, passing the mixture through an inclined pipe        having an inlet-at its upper end that is connected to the outlet        of the supply pipe, whilst maintaining a stratified flow in the        inclined pipe so that the phase of fluid of lower density        (“lighter phase”) is above the phase of fluid of higher density        (“heavier phase”);    -   extracting fluid with the lower density via a first discharge        system having an inlet that is located such that is in fluid        communication with the lighter phase;    -   extracting fluid of higher density via a second discharge system        located at the inclined pipe and having an inlet that is in        fluid communication with the heavier phase;        wherein the interface between the lighter phase and the heavier        phase is monitored in the inclined pipe by a level controller        means that varies the flow of the fluid of higher density to        keep the interface between set levels.

The method provides an easy way to ensure that the different phases arewithdrawn separately from the separation device. The fact that themixture is maintained in a stratified flow is a major enabler of thisaccomplishment. In one embodiment the supply pipe is different from thepipeline conduit through which the mixture is fed to the separationdevice described. This is suitably the case when the mixture is not in astratified flow, and the supply pipe serves then to achieve suchstratified flow. In an alternative embodiment, in particular when themixture is already in a stratified flow, the supply pipe is the same asthe pipeline conduit. Applicant had found that stratified flow in thesupply conduit can be maintained if the diameter of the supply pipe isselected such that during normal operation the velocities of the liquidsare below a pre-determined value, and if the ratio of the length of thesupply pipe to its diameter is larger than 5, especially larger than 10.Applicant has further found that the slope of the inclined pipe can beselected such that during normal operations a stratified flow ismaintained in the inclined pipe. Suitably, the slope of the inclinedpipe ranges between 1 and 5, preferably between 1 and 3° from thehorizontal plane. The stratified flow makes it possible to use a levelmonitor to determine where the interface between the heavier and thelighter phases is. Via the first or second discharge system the levelcan then be adjusted. Preferably one employs a level controller incombination with a valve as the level controller means in the firstand/or second discharge system.

The second discharge system must be in fluid communication with theheavier phase in the inclined pipe. A skilled artisan will realize thatthe location of this discharge system can be along any place of theinclined pipe. Most conveniently, the second discharge system is at thedownstream end of the inclined pipe.

It is a further embodiment of the present invention to provide aseparation device that can be used in the current method. Accordingly,the present invention also provides a device for separating a mixture offluids into at least two phases, one of which has a higher density thanthe other, so that a phase of fluid of lower density (“lighter phase”)and a phase of fluid of higher density (“heavier phase”) are obtained,which device comprises:

-   -   a normally horizontal supply pipe with a feed inlet at its        upstream end and an outlet at its downstream end;    -   an inclined pipe having an inlet at its upper end that is        connected to the outlet of the supply pipe;    -   a first discharge system having an inlet that is located such        that is in fluid communication with the lighter phase; and    -   a second discharge system located at the inclined pipe and        having an inlet that is in fluid communication with the heavier        phase,        wherein the inclined pipe is provided with a level controller        means comprising a level monitor and a valve at the first and/or        second discharge system.

A skilled artisan will realise that many types of level monitors can beused. Examples of such level monitors include floating devices orsegmented monitors. Other examples are dip sticks, magnetic level liquidindicators, conductivity- or capacitance-based devices and ultrasoniclevel devices. An overview of suitable devices has been described inKirk-Othmer, Encyclopedia of Chemical Technology, John Wiley, New York,4th ed. Vol. 15, 1995, pp 409-433. The level monitors and the valves maybe operated via a computer-directed system.

The invention will now be described by way of example in more detailwith reference to the accompanying drawings, wherein

FIG. 1 shows schematically a first embodiment of the invention, and

FIG. 2 shows schematically a-second embodiment of the invention.

Reference is made to FIG. 1. A device 1 for separating a mixture offluids in two phases is shown. It contains a horizontal supply pipe 2.At the upstream side thereof it has an inlet that is connected to apipeline 3 that feeds the device with the mixture. It is noted thatin-this embodiment the supply pipe 2 ensures a stratified flow that isnot present in pipeline 3. At its downstream end it is connected to aninclined pipe 4. In the pipe 2 a stratified flow is created and ininclined pipe 4 such stratified flow is maintained so that two distinctphases are apparent; one lighter phase of fluid with a lower density anda heavier phase with a fluid of higher density. The device 1 alsocomprises a first discharge system 5, the inlet of which is in fluidcommunication with the lighter phase. The discharge systems results in adischarge pipe 6 through which the low-density fluid is discharged. Theinclined pipe 4 ends in a second discharge system 7 that boils down toan outlet through which the high-density fluid is passed into adischarge pipe 8. The interface between the two phases is monitored viaa level controller 9. If the level controller 9 indicates that theinterface goes beyond preset levels, it causes a valve 10 to adjust theflow of higher-density fluid through the discharge pipe 8, in order tobring the interface back to the preset level range. The valve 10 ispositioned in the discharge pipe 8. It is also possible to place thevalve at the outlet of the inclined pipe or at the lower end of theinclined pipe.

In an embodiment where the flow rate of higher-density fluid isrelatively high in comparison to the flow of the low-density fluid, itis suitable to extract the fluid of higher-density via a furtherdischarge system having a further outlet (11) that is in fluidcommunication with the heavier phase. Preferably, the further outlet 11is arranged in the bottom of the horizontal supply pipe. Thehigher-density fluid is then suitably withdrawn via the outlet 11through a discharge pipe 12. Suitably, this flow of higher-density fluidis combined with the higher-density fluid withdrawn from the seconddischarge system 7 in discharge pipe 8. It is preferred to have thevalve positioned in the second discharge system. An additional controlmeans is suitably arranged such that the pressure of the lighter phasein the first system is monitored, and the flow of fluid of lower densityis adjusted in accordance with the pressure measured. Although it ispossible to arrange the flow adjustment of the heavier phase inaccordance with the pressure measured, it is preferred from a pragmaticviewpoint to adjust the flow of low-density fluid.

The device and method are suitable employed in the production ofhydrocarbons, in which production also water is recovered. Hence thelighter phase in the present-method comprises suitably oil, and theheavier phase comprises suitably water. In a number of occasions thewell from which hydrocarbons are produced yield liquid hydrocarbon oil,water and gas. In such instances it is advantageous to separate thewater from the liquid and gaseous hydrocarbons. Thereto, the presentinvention suitably provides a method in which three phases are separatedinto a phase of fluid of lower density (“lighter phase”), a phase withfluid of intermediate density (“intermediate phase”) and a phase withfluid of higher density (“heavier phase”). The lighter phase suitablycomprises gas, the intermediate phase comprises oil and the heavierphase comprises water. Although in principle it is possible to withdrawthe combination of the intermediate and the heavier phases together viathe second discharge system, it is preferred to withdraw the lighterphase and the intermediate phase via the inlet of the first dischargesystem. Reference is made to FIG. 2, wherein the device 20 comprises ahorizontal supply pipe 21 and an inclined pipe 22. A mixture of gas, oiland water is fed to the device 20 via a pipe 23. A riser 24 is in fluidcommunication with the lighter and the intermediate phases, i.e. gas andoil. The lighter and intermediate phases are withdrawn from thehorizontal pipe, and the withdrawn phases are suitably passed to theriser section 24 to allow the lighter phase and the intermediate phaseto separate. The combined phases may be withdrawn from the riser section24 in any known manner. One can pass the combined phases directly to adischarge conduit (not shown). It is also possible to withdraw thelighter phase from the upper part of the riser section and theintermediate phase from a lower part of the riser section (not shown).It is preferred to provide a riser section that comprises two vessels,and pass the combination of the lighter and intermediate phases from afirst vessel 24 to a second vessel 25. Vessel 25 can have the shape of avessel as shown in the figure, but it will be clear that also othershapes, such as a short (horizontal) pipeline section can be used. Inthis second vessel 25 the phases clearly separate. The lighter phase isthen withdrawn from the top of the second vessel 25 via a conduit 27,and the intermediate phase is withdrawn from the bottom of the secondvessel 25 via a conduit 26. Suitably, the interface between the lighterphase and the intermediate phase is monitored by a second levelcontroller means 28 that adjusts the flow of the fluid of intermediatedensity to keep the interface between set levels using a valve 29arranged in the discharge pipe 26. Although not necessary, it issometimes economic to combine both phases from pipes 26 and 27 forfurther transport.

In this method the second level controller 28 is arranged at the secondvessel 25 to monitor the interface between the lighter and intermediatephases. Suitably, the level controller 28 communicates with a valve 39,arranged in the conduit 26 for the fluid of intermediate density, tocontrol the flow of fluid of intermediate density. The skilled artisanwill realise that it is possible to arrange a similar valve in thedischarge conduit 27 to control the flow of fluid of lighter density,and have that valve governed by the level controller 28. An alternativeembodiment is to have one or more similar level controllers communicatevalves in both discharge pipes to control both flows. It is furtheradvantageous to arrange a pressure monitor 29 at the discharge pipe 27that communicates with one or more flow control valves (30) in the firstdischarge system for control of the flow of the fluid of lower density.Alternatively, the pressure controller 29 may communicate with a valvein discharge pipe 26 to control the flow of fluid of intermediatedensity. A further embodiment provides communication of the pressurecontroller with two valves in pipes 26 and 27 to control both flows. InFIG. 2 only a valve 30 is shown in the discharge conduit 27 for thefluid of lower density. In this way the pressure in the riser section(24,25) is monitored and the flow of the fluid of lower density, or theflow of fluid of intermediate density or both flows are adjusted inaccordance with the pressure measured. The high-density fluid, e.g.water, is withdrawn from the inclined pipe 22 via a discharge pipe 31.The level between the intermediate and heavier phases is monitored by alevel controller 32 that is in communication with a valve 33, arrangedin discharge pipe 31. The system of level controller 32 and valve 33operates similarly to the equivalent system described for FIG. 1.

The following is an example of the separation device wherein referenceis made to the embodiment of FIG. 2.

A mixture of gas, oil and water is fed via a tube of 10″ (254 mm)internal diameter to the separation device 20. The horizontal supplypipe 21 and the inclined pipe 22 have an internal diameter of 48′ (1.2m). Therefore the superficial liquid velocity in the device is reducedby a factor of about 24. The length of the horizontal supply pipe andthat of the inclined pipe may range from 12 to 100 m, and from between15 and 120 m, respectively. It is again emphasised that the length ofthe supply pipe is not relevant as long as a stratified flow is createdin the supply pipe. If the mixture id fed through a pipeline conduit ina stratified flow, the supply pipe can be as long as many tens ofkilometres. The temperature of the mixture varies between 60 to 85° C.and the pressure may be varied between 25 and 85 bar. Suitably the levelmonitor is in the middle of the inclined section. That could be at about10 m downstream of the riser section. Under the conditions describedabout 10,000 to 20,000 m³ of the mixture per day can be passed throughthe separation device.

1. A method for separating a mixture of fluids into at least two phases,one of which has a higher density than the other, which methodcomprises: passing the mixture through a normally horizontal supply pipewith a feed inlet at its upstream end and an outlet at its downstreamend; subsequently, passing the mixture through an inclined pipe havingan inlet at its upper end that is connected to the outlet of the supplypipe, whilst maintaining a stratified flow in the inclined pipe so thatthe phase of fluid of lower density (“lighter phase”) is above the phaseof fluid of higher density (“heavier phase”); extracting fluid with thelower density via a first discharge system having an inlet that islocated such that is in fluid communication with the lighter phase;extracting fluid of higher density via a second discharge system locatedat the inclined pipe and having an inlet that is in fluid communicationwith the heavier phase; and wherein the interface between the lighterphase and the heavier phase is monitored in the inclined pipe by a levelcontroller means that varies the flow of the fluid of higher density tokeep the interface between set levels.
 2. The method of claim 1, whereinthe level controller means comprises a level monitor and a valve at thefirst and/or second discharge system
 3. The method of claim 1, whereinthe supply pipe is the same as the pipeline conduit through which themixture to be separated is fed.
 4. The method of claim 1, wherein thefluid of higher density is extracted via a further discharge systemhaving a second outlet that is in fluid communication with the heavierphase.
 5. The method of claim 1, wherein the second outlet is arrangedin the bottom of the horizontal supply pipe.
 6. The method of claim 1,wherein the pressure of the lighter phase in the first discharge systemis monitored, and the flow of fluid of lower density is adjusted inaccordance with the pressure measured.
 7. The method of claim 1, whereinthe lighter phase comprises oil; and the heavier phase comprises water.8. The method of claim 1, wherein three phases are separated into aphase of fluid of lower density (“lighter phase”), a phase with fluid ofintermediate density (“intermediate phase”) and a phase with fluid ofhigher density (“heavier phase”).
 9. The method of claim 1, wherein thelighter phase comprises gas, the intermediate phase comprises oil andthe heavier phase comprises water.
 10. The method of claim 1, whereinthe lighter phase and the intermediate phase are withdrawn via the inletof the first discharge system
 11. The method of claim 1, wherein thewithdrawn phases are passed to a riser section to allow the lighterphase and the intermediate phase to separate.
 12. The method of claim 1,wherein the lighter phase is withdrawn from the riser section at theupper level of the riser section, and the intermediate phase iswithdrawn from the riser section at the lower level of the risersection.
 13. The method of claim 1, wherein the riser section comprisestwo vessels.
 14. The method of claim 1, wherein the interface betweenthe lighter phase and the intermediate phase is monitored by a secondlevel controller means that adjusts the flow of the fluid of lowerdensity to keep the interface between set levels.
 15. The method ofclaim 1, wherein the second level controller means communicates withvalves that control the flow of the fluid of lower density or the flowof the fluid of intermediate density or both flows.
 16. The method ofclaim 1, wherein the pressure in the riser section is monitored and theflow of the fluid of lower density, or the flow of fluid of intermediatedensity or both flows are adjusted in accordance with the pressuremeasured.
 17. A device for separating a mixture of fluids into at leasttwo phases, one of which has a higher density than the other, so that aphase of fluid of lower density (“lighter phase”) and a phase of fluidof higher density (“heavier phase”) are obtained, which devicecomprises: a normally horizontal supply pipe with a feed inlet at itsupstream end and an outlet at its downstream end; an inclined pipehaving an inlet at its upper end that is connected to the outlet of thesupply pipe; a first discharge system having an inlet that is locatedsuch that is in fluid communication with the lighter phase; and a seconddischarge system located at the inclined pipe and having an inlet thatis in fluid communication with the heavier phase, wherein the inclinedpipe is provided with a level controller means comprising a levelmonitor and a valve at the first and/or second discharge system.
 18. Thedevice of claim 17, comprises a further discharge system having a secondoutlet that is in fluid communication with the heavier phase.
 19. Thedevice of claim 17, wherein the second outlet is arranged in the bottomof the horizontal supply pipe.
 20. The device of claim 17, wherein thefirst discharge system is provided with a pressure monitor thatcommunicates with a flow control valve located in the first dischargesystem
 21. The device of claim 17, wherein the first discharge systemcomprises a riser section to allow the lighter phase and a phase withfluid of intermediate density (“intermediate phase”) to separate, whichriser section has an outlet for the lighter phase in the upper part andan outlet for the intermediate phase in the lower part.
 22. The deviceof claim 17, wherein the riser section comprises two vessels.
 23. Thedevice of claim 17, wherein the riser section has been provided with asecond level controller means that adjusts the flow of the fluid oflower density to keep the interface between set levels.
 24. The deviceof claim 17, wherein the second level controller means communicates withone or more valves at the respective discharge systems, which valve(s)control(s) the flow of the fluid of lower density or the flow of thefluid of intermediate density or both flows.
 25. The device of claim 17,wherein the riser section is provided with a pressure monitor thatcommunicates with one or more flow control valves for control of theflow of the fluid of lower density, or the flow of fluid of intermediatedensity or both flows.